1. Field of the Invention
The present invention relates to a rotary encoder capable of detecting rotational information such as a rotational speed or a rotational angle of a rotating object with high accuracy.
2. Description of the Related Art
Rotary encoders are often used as detectors for detecting rotational information of a rotating member used in machine tools and FA apparatuses, as disclosed in Japanese Patent Laid-Open Nos. 2006-214929 and 06-18285. Angle detection methods using the rotary encoder are roughly divided into an incremental method that detects relative rotational information and an absolute method that detects absolute rotational information. The incremental method using an optical encoder disclosed in Japanese Patent Laid-Open No. 2006-214929 simplifies the configuration of the encoder. However, the incremental method has problems that a power-off may cause a loss of the rotational information and that detection errors caused due to an exogenous noise may accumulate.
On the other hand, the rotary encoder for the absolute method generally has advantages that highly accurate detection is easily performed since the detection errors caused due to the exogenous noise do not accumulate and that a movement to a home position is not necessary when the power is off. An absolute rotary encoder disclosed in Japanese Patent Laid-Open No. 06-18285 uses a spiral scale in which a detection light-transmissive portion is formed spirally around a rotation center, the spiral scale being formed in a rotating plate attached to a rotating axle. In this absolute rotary encoder, of light fluxes emitted from an LED, a light flux transmitted through the detection light-transmissive portion in the spiral scale is detected by a detection optical position sensitive detector (hereinafter referred to as a “detection PSD”).
Since the detection light-transmissive portion is spirally formed, a radial entrance position of the light flux entering the detection PSD continuously changes according to a rotational angle θ of the rotating plate (rotating axle). The detection PSD outputs a signal corresponding to the radial entrance position of the light flux, and thereby the output signal (detection light-receiving signal) of the detection PSD is linearly changed with the rotational angle θ. Therefore, calculation of the rotational angle θ of the spiral scale (that is, of the rotating axle) can be made based on the detection light-receiving signal. In addition, a light flux transmitted through a reference light-transmissive portion formed circularly centering on the rotation center in the rotating plate is received by a reference PSD. Subtracting a light-receiving signal output from the reference PSD from the light-receiving signal output from the detection PSD makes it possible to eliminate influences of decentering and backlash of the spiral scale with respect to the rotating axle.
Furthermore, there is a discontinuous area in the spiral detection light-transmissive portion near a circumferential position corresponding to a rotational angle of 0 degree in the rotating plate disclosed in Japanese Patent Laid-Open No. 06-18285. In this discontinuous area in the spiral detection light-transmissive portion, the reference PSD is disposed such that a light flux transmitted through a recognition light-transmissive portion (recognition area) formed in the reference light-transmissive portion may be detected by the reference PSD. Entrance of the light flux transmitted through the recognition light-transmissive portion into the reference PSD increases a light-receiving quantity of the reference PSD, which makes it possible to recognize that the rotating plate is rotated in the recognition area corresponding to the discontinuous area and then perform a calculation process for the discontinuous area. The rotational angle θ is thus detected over an entire rotational angle range of the rotating plate.
In the rotary encoder disclosed in Japanese Patent Laid-Open No. 06-18285, an arrangement error of each PSD and a variation of ambient temperature may cause a detection position displacement between the discontinuous area of the spiral detection light-transmissive portion and the recognition area of the reference light-transmissive portion, which results in an angle detection error near the discontinuous area. For example, in a case where the reference PSD is disposed with a positional error with respect to the detection PSD in a rotation direction, detection of the recognition area does not match the position of the discontinuous area, which causes an incorrect calculation process. Moreover, the temperature variation varies a determination threshold level, which causes a problem of mismatch between the detection of the recognition area and the position of the discontinuous area.